This invention relates to a dock leveler, and more particularly to a dock leveler having a ramp movable from a lowered position to a raised position and having a lip pivotably mounted to the ramp for movement between an inoperative, pendant position to an operative raised position as the ramp moves from its raised position to its lowered position.
U.S. Pat. No. 3,997,932, issued Dec. 21, 1976 and assigned to the same assignee as the present application, discloses a lip lifting mechanism for a dock leveler for moving the lip of the dock leveler from an inoperative pendant position to an operative raised position as the ramp is lowered from its raised position toward its lowered position. This extension of the lip during downward movement of the ramp enables the lip to engage the bed of a truck or the like parked adjacent the dock, to facilitate loading or unloading of the truck. Generally, the lip lifting mechanism disclosed in U.S. Pat. No. 3,997,932 includes a push bar pivotably mounted at its rear end to the frame of the dock leveler within the pit of the dock within which the dock leveler is mounted, and a lifting rod pivotably interconnected with the lip via pivoting lug structure. The forward end of the lifting rod is pivotably mounted to the lug structure. The rearward end of the lifting rod is pivotably mounted toward one end of a pair of links, which are pivotably mounted at their opposite end to a hanger bracket mounted to and extending downwardly from the deck plate of the ramp. A latch pin extends between the links, and is engageable by a notch formed in the push bar as the ramp is lowered. The push bar, links, latch pin and lifting rod define a toggle mechanism in which the lifting rod and links are moved by engagement of the push bar notch with the latch pin to an over-center toggle position upon lowering of the ramp, to raise and latch the lip in its operative position as the ramp is lowered by engagement of the lug with the lip. Once the lip engages the truck bed, the weight of the lip exerted on the lifting rod through the lug is relieved, to break the toggle by allowing the link and lifting rod to pivot to an under-center, inoperative position. When the lifting rod and link are in the over-center toggle position, the second end of the push bar is moved upwardly so as to disengage the push bar notch from the latch pin, thus enabling the link and lifting rod to fall to the under-center position when the lip rests on the truck bed. When the ramp is raised, the lip is free to fall to its inoperative pendant position, in preparation for a subsequent cycle for again moving the lip to its operative extended position as the ramp is raised and lowered.
It is an object of the present invention to provide a lip lifting mechanism having generally the same construction and operation as summarized above, yet which provides improved and advantageous operation. A still further object of the invention is to ensure that the lip is at all times returned to its inoperative position when the ramp is in its fully lowered position, even when the lip does not engage the truck bed as the ramp is moved toward its lowered position. Yet another object of the invention is to provide a lip lifting mechanism which also functions as a stop to limit upward movement of the ramp to its raised position.
In accordance with one aspect of the invention, an improved lip lifting mechanism for a dock leveler includes a push bar defining first and second ends, with the first end being pivotably mounted to a stationary structure, such as the dock leveler frame located within the pit toward its rearward end. An axial lifting rod defines first and second ends, with a first end of the rod being pivotably mounted to a pivotable lug with which the lip is engageable. One or more links, preferably a pair of links, are pivotably mounted to the lifting rod toward its second end for pivoting movement about a first pivot axis. The links are also pivotably mounted to the ramp, such as to a hanger bracket mounted to the under-side of the deck plate of the ramp, for pivoting movement about a second pivot axis. The second end of the push bar is located adjacent the links and movable relative to the links upon movement of the links caused by raising and lowering of the ramp. Engagement structure is interposed between the links and the push bar for engaging the push bar with the links after the ramp has been raised and during downward movement of the ramp toward its lowered position. The engagement structure functions to move the links and the lifting rod into an over-center toggle position as the ramp moves toward its lowered position, to move the lip to its extended latched and operative position. The engagement structure between the links and the push bar is engageable at a location spaced toward the push bar from a transverse axis interconnecting the first and second pivot axes of the links, to increase the force exerted by the push bar on the lifting rod over an arrangement in which the engagement structure is in line with the transverse axis interconnecting the first and second pivot axes of the links. The hanger bracket defines a forward edge and a downwardly facing lower edge. The engagement structure includes a latch pin, such as in the form of a bolt extending between the pair of links, with the latch pin being engageable with the lower edge of the bracket to limit pivoting movement of the links and to provide a stop to fix the links and the lifting rod in the over-center toggle position in which the lip is moved to its extended latched operative position prior to engagement with the truck bed. The lower edge of the bracket defines a notch toward its forward end, within which the latch pin is received when the latch pin and the lifting rod are in the over-center toggle position. The latch pin is mounted to the links such that the latch pin is located below the transverse axis interconnecting the first and second pivot axes of the link, when the link and the lifting rod are in the over-center position. The location of the latch pin, in combination with an angled engagement surface provided on the push bar to engage the latch pin, exerts a significantly increased force on the latch pin by the push bar during descent of the ramp. The forward edge of the bracket further includes a notch, which receives a stop pin mounted toward the second end of the push bar, so as to limit upward movement of the ramp to its raised position.
In accordance with another aspect of the invention, a release arrangement is provided for moving the link and lifting rod from the over-center toggle position when the ramp reaches a predetermined point in its descent from its raised position to its lowered position. The release arrangement includes a trip member mounted to the ramp and movable therewith, which is engageable with the second end of the push bar as the ramp approaches its fully lowered position. This causes engagement of the push bar with a roller mounted coaxially with the first pivot axis about which the links are pivotably mounted to the lifting rod, which breaks the toggle formed by the links and lifting rod when in the over-center toggle position. The trip member is preferably in the form of a downwardly extending angle section mounted to the under-side of the ramp deck plate. This construction provides positive movement of the links and lifting rod to an under-center position in which the toggle is broken, so as to ensure that the lip falls to its pendant position at the completion of each cycle, even when the lip does not engage a truck bed during downward movement of the ramp from its raised position toward its lowered position.
Various other features, objects and advantages of the invention will be made apparent from the following description taken together with the drawings.